This invention relates to an improved process for polymerizing monomers derived from substituted or unsubstituted acrylic acid/methacrylic acid and esters thereof using an initiating amount of a tertiary alkyl hydroperoxide having at least five carbons and/or its derivatives wherein the polymer product has a low molecular weight and a narrow molecular weight distribution suitable for high solids coating applications.
There is a need in the coatings industry to develop polymers which possess a narrow molecular weight distribution (MWD) for use in high solids coating formulations. Such polymers must not only have a low molecular weight and a low viscosity in order to produce a sprayable solution, but must also contain chemically active groups (usually hydroxyl or carboxyl functionality) in order to undergo molecular weight build up and network formation during the crosslinking (i.e., curing) reaction. A more uniformed, homogeneous crosslinked network can be produced if the polymer possesses a narrow molecular weight distribution (MWD). Hence, a narrow MWD improves overall film properties and influences the viscosity of the coating solution. Therefore, it is desirable for polymers used in high solids coating formulations to be of low molecular weight and to prossess a narrow MWD.
Free radical solution polymerization is the most widely used commercial process for the preparation of polymers suitable for use in high solid coating formulations. Azonitrile compounds, especially symmetrical azonitrile compounds, are currently being used as the free radical source in the industry for producing polymers suitable as high solids coating resins by solution polymerization technique. Azonitrile initiators generally produce much narrower MWD polymers in comparison to conventional organic peroxide initiators. (Conventional organic peroxides are primarily derivatives of tertiary butyl hydroperoxides). By the ability of the azonitrile compounds to produce narrow MWD polymers, this makes azonitrile compounds preferred initiators for high solid coating resin production.
Although tertiary alkyl peroxide having at least five carbon atoms are known in the prior art, no publication was found which discloses the use of these peroxides for producing polymers having a narrow molecular weight distribution which polymers will be suitable in high solids coating formulations. U.S. Pat. Nos. 3,686,102, 3,950,432, and 4,137,105 discloses the use of tertiary amyl(C.sub.5) and tertiary octyl (C.sub.8) peroxides as free radical initiators for vinyl polymerization. Also, U.S. Pat. No. 4,130,700 discloses the use of tertiary amyl diperoxyketals as finishing catalysts to reduce residual styrene levels for the bulk polymerization of styrene.